VOL. 3, No. 3
C~NSTRUCTING MODELS OF CARBON COMP~~NDS
319
A DEVICE FOR CONSTRUCTING MODELS OF CARBON COMPOUNDS W.H. DORE, UNIVERSITY OF CALIPORNIA, BERKELEY, CALIFORNIA I n teaching organic chemistry some form of solid model to demonstrate the space relationships of the atoms in the molecule is indispensable. Such models, however, are not used as frequently as is desirable because of the expense and trouble that are involved in their construction. To help overcome that difficulty, the author offers this description of a con-
venient and inexpensive method for preparing stereo-chemical models. The materials are readily obtainable and the construction work can be done by students or by anyone who is handy with tools. The conventional system of balls and wires is used. Plain wooden balls of about 30 rnm. diameter are obtainable from novelty stores or mills for about one dollar per hundred. Holes are bored in these and the balls are joined by stiff steel wire (No. 18, B. & S. wire gauge = 0.04"). Probably the most perplexing detail in the construction of these models is boring the holes in the balls representing the carbon atoms a t the proper
angles to give the tetrahedral structure. This diffculty may be readily overcome by following the directions which are given below. Obtain a piece of board about six inches square and saw it, as shown in Fig. 1, on the lines ab and cb. The angle abc should be exactly 109.5" and should be bisected by the line db which is perpendicular to the top line of the drawing. The board is thus cut into two pieces so that db = be.
FIG.2.
Next cut out the notches M and N so that df and ge are equal and the notches are equal in width to the thickness of the board. Assemble the boards by dove-tailing the notches together as shown in Fig. 2. Nail or screw the pieces to a baseboard to secure stability. Then bore a vertical hole a t the intersection of the pieces, using a bit having a diameter slightly larger than that of the balls which are to be used. The hole should be bored about one inch deep; care should be
taken not to get too near the junction of the two notches and so weaken the dove-tail. The dotted line and arrow in Fig. 2 show where the hole is to be bored; the larger circle in Fig. 3 shows how it appears from above while Fig. 4 shows i t in cross-section. The four edges, A, B, C, and D, of the intersecting pieces will take the direction of the tetrahedral bonds. If a ball be placed in the cylindrical bore and be supported a t just the right height, the surfaces, A, B, C, and D, will serve as guides for boring the holes and placing the wires. In order to adjust correctly the position of the ball in the cylindrical bore, screw a large headed wood screw in the center of the hole as shown in the crosssection (Fig. 4). This screw should be so set that the center of the ball
comes a t the imaginary intersection point of the projected planes, A , B, C, and D. Four small nails should be placed on each of the planes, A and B, as shown in Fig. 5, their purpose being to wide the boring tool and wires. It has been found convenient to first mark each hole rather deeply with an ice pick, held on A as shown in Fig. 5, then remove the ball and bore it with a drill bit which is held in the chuck of a lathe while the ball is pressed against it. One hole is bored as described and a short wire is inserted in the hole. The ball is then placed in the pocket again with the wire extending along plane B and between the nails. With its location and direction thus fixed, another hole is marked by the pointer, resting it on plane A. After boring the hole a wire is inserted in this hole also. The ball is again placed in the pocket, this time with the wires lying along the central lines of planes C and D. The remaining holes are then marked by the pointer resting i t on planes A and B. The balls representing oxygen, as well as carbon atoms, should be
bored in this manner in order to conform to the modern conception of tetrahedrally placed valences for oxygen. Only two wires, however, are used to represent the primary valences. If desired, the other two points may be marked and brass headed tacks driven in to indicate the probable location of residual valences. Hydrogen has, of course, only
one primary and no secondary valence, consequently it is represented by a single wire. It has been found useful to have the wires of such length as to keep the distance between the balls proportional to their atomic radii. (For a table of atomic radii in Angstrom units, see Science, 61, 544, May 29, 1925.) A convenient-sized model is obtained if the balls are spaced so that their centers are separated by as many millimeters as there are hun-
dredths of an h g s t r o m unit in the average of their atomic radii. For example, in uniting carbon and oxygen, whose radii are 0.73 A. U. and 0.65 A. U., respectively, the average, 0.69 A. U., would establish the spacing between centers as 69 mm. By adhering to this scale, it becomes possible .to compare X-ray spectrum data with measurements upon the model. To avoid confusion of the different atoms, the balls should be painted, each element having a characteristic color. The author has used black for carbon, red for oxygen, and white for hydrogen. If these are varnished after painting, the model is given an attractive appearance and photographs well. For photographic purposes, however, the red balls reproduce much like the black ones. By painting white bands or white spots on the red balls, they become readily distinguishable in the photographs.
